The present invention relates generally to a transmission type illumination device for stereomicroscopes and a stereomicroscope, and more particularly to a transmission type illumination device for a stereomicroscope that can incorporate a viewing optical arrangement comprising a pair of left and right zooming optical systems toward the front of a microscope or a viewing optical system comprising an objective lens common to a pair of left and right zooming optical systems to view stereoscopic images.
Transmission type illumination devices for stereomicroscopes known so far in the art comprise an optical element having a periodical structure in a one-dimensional direction (hereinafter called the prism sheet) or a similar member for the purpose of making improvements in illumination variations and illumination efficiency, as typically set forth in the following patent publications 1 and 2.
Patent Publication 1
JP-U 51-82466
Patent Publication 2
JP-A 5-40226
For stereomicroscopes, there is now a growing demand toward having a much wider range of variable viewing magnification. FIGS. 5(a), 5(b) and 5(c) are illustrative of how to vary magnification by means of a zoom lens (not shown). That is, there are shown optical paths of back ray tracing for the light beams necessary for illumination of the whole field of view from an objective lens 6 to a diffuser 3 in the illumination system. Reference numeral 1 stands for a light source, 2 a collector lens, 3 a diffuser, 4a a convex lens located in the vicinity of the diffuser 3, 4b a convex lens located in the vicinity of a viewing surface 5, 5 a viewing surface (or viewing position), 6 an objective lens, p a pupil of a viewing system, and p′ a positive conjugate to p.
As can be seen from FIG. 5, the position p′ conjugate to the pupil p of the viewing system and its magnitude change largely with a magnification change by the zoom lens located in the viewing optical system, and the angle of incidence on the diffuser 3 of a light beam arriving at each point on the field of view varies. Generally, the reason the magnitudes of the pupil p and p′ vary for each magnification is that as the magnification of the viewing optical system becomes high, its angular aperture becomes large.
The above phenomena become much more noticeable when this zoom lens is used in combination with the objective lens 6 having a different focal length to enlarge the range of magnification where images can be viewed. Until now it is still difficult to achieve illumination with reduced field variations by means of a single arrangement while light beams are allowed to meet the requirements for the pupil p and p′ over such a wide magnification range.
FIG. 6(a) is illustrative of optical paths from a light source 1 to an objective lens 6-100 in a stereomicroscope on which the objective lens 6-100 having a long focal length (f=100 mm) is mounted. As shown, there is provided a collector lens 2 for converting light from the light source 1 to a generally parallel light beam, a diffuser 3 for receiving a light beam from the collector lens 2 to form a secondary light source of surface shape, and a convex lens system comprising a convex lens 4a located near the diffuser 3 for converting a light beam diverging from the diffuser 3 into a converging light beam to be directed to an object on a viewing surface 5 and a convex lens 4b located near the viewing surface 5.
When the objective lens 6-100 has a long focal length (f=100 mm) as shown in FIG. 6(a), there is no shading because a light beam reaching the periphery of the field of view of the viewing surface 5 is guided to the diffuser 3 as viewed in back ray tracing. When an objective lens 6-50 having a short focal length (f=50 mm) is provided as shown in FIG. 6(c), however, shading occurs because a light beam reaching the periphery of the field of view of a viewing surface 5 deviates from the illumination optical system, as viewed in back ray tracing. Referring here to FIG. 6(b) wherein an objective lens 6-75 has an intermediate focal length (f=75 mm), there is again no shading because a light beam reaching the periphery of the field of view of a viewing surface 5 is guided to a diffuser 3 as viewed in back ray tracing.